It is common knowledge that petroleum products extracted from a well is stored on a floating storage facility or on a subsea tank. It is also known that it may be a Floating Production, Storage and Off-loading vessel (commonly known as FPSO) or an Offshore Storage Unit (OSU) which are employed to temporarily store oil received from the well. Subsequently, the oil is transferred to an oil-tanker or similar to bring it to the shore.
The mooring system is a critical component of a floating production platform, and its integrity over the field life is of paramount importance. Where the platform has a drilling or workover rig, an active mooring system enables the platform to position itself over the well.
Spread mooring consists of multiple legs that are connected to the platform by fairleads and tensioners and to the seabed by anchors. In deep water, each leg is typically made up of either steel wire or synthetic rope over most of its length, with a small segment of chain at the top and bottom. The mooring spread could be symmetrically arranged or grouped around the platform. The mooring legs must resist forces and motions induced by the platform in response to extreme weather conditions. Design codes specify safety factors for line break strength and fatigue life that are not to be exceeded by loads induced in the mooring legs.
Mooring systems resist steady environmental loads by generating a restoring force created by unbalanced horizontal tensions of the mooring array when the vessel offsets from its mean position.
It is of utmost importance that there must be appropriate mooring units, including chain stopper, to ensure optimal positioning of the vessel.
OPB fatigue occurs where two chain links are in contact and when one of the links rotates relative to the other, introducing bending moments. The reason for these rotations is that the moored vessel is constantly moving due to waves, wind and currents in relation to the anchor, which is fixed on the seabed. When this happens, the chain angle between the anchor and the vessel changes.
These changing chain angles introduce bending moments in the chain, which may over time break the chain due to fatigue. The present invention aims to reduce these interlink rotations by introducing a universal joint designed to take the constantly changing movement. In principle, one end of the universal joint is attached to the vessel and the other part to the mooring chain.
Additionally it shall be possible to pull the chain through the unit for installation, pre-tensioning, chain replacement and decommissioning.
There are a number of patents relating to the out of plane bending (OPB) challenges.
Publication U.S. Pat. No. 7,926,436 shows a dual chain axis stopper, which has a universal joint that is pivotally coupled to a vessel in one end and a mooring unit arm in a second perpendicular end. The coupling are situated in two different planes defines by axes extending through two oppositely arranged shaft on the universal joint. The axes are situated in two different planes defined by shafts in the universal joint that are coupled to the vessel or mooring unit arm. The shaft in the upper part of the universal joint are coupled to the vessel and the shafts in the lower part of the universal joint are coupled to the mooring unit arm. Along the mooring unit arm, at a distance from the lower axis, there is arranged a chain stopper. This distance between the chain stopper and the bearings of the axes makes the rotation of the universal joint as easy as possible and will overcome the friction the bearing. The disadvantage with the arrangement of the publication is that the mooring unit and hence the chain will primarily move about the shafts in the two perpendicular directions. When the universal joint is rotated in one direction this could cause the chain to contact the mooring unit that could cause fatigue problems in the chain and especially out of bend fatigue problems. The rotation mooring in two planes is also disadvantageous because the bending moments of the brackets are higher when the direction of the rotation is restricted. The chain support according to the invention is advantageous over prior art in that the invention reduces the out of plane fatigue problems as the dual chain stopper according to the invention allows movement in the same plane. This provides further a mooring arrangement that is adapted to bend in approximately any directions since the axes are arranged in substantially the same plane. This will thus reduce fatigue problems that can occur due to movement of the vessel in relation to the mooring arrangement attached to the seabed. It is also possible to pull the chain through the chain support for installation, pre-tensioning, chain adjustments, chain replacement and decommissioning.
The drawbacks in the prior art publications have triggered the need for a mooring unit that will reduce the bending forces acting on the connection point on the vessel, for example the bracket.